Two of the characteristics of a golf club that are paid particular attention by golf aficionados are its moment of inertia, commonly known in the golfing world as the "swing weight" of the golf club, and its center of gravity) (COG). The swing weight and center of gravity are the main determinants of the "feel" and performance of the golf club.
The swing weight of a golf club is defined by the mass of the club at each location distant from the location where the golf club is gripped. A club's swing weight generally depends most heavily on the mass and configuration of the golf club head since the club head is the heaviest portion of the club and is located most distantly from the grip.
The club's center of gravity can be defined as that point within the club's volume about which the club's weight is evenly distributed; in other words, the club is "balanced" about its center of gravity. Technically, aficionados are not so much concerned with the center of gravity of the entire club, but are more concerned with the center of gravity of the club head. The location of the center of gravity within the club head is of importance because it will affect the behavior of the club on impact, and will thus affect the flight of the golf ball. If the center of gravity is situated near the toe or heel of the club head, this will affect the tendency of the club to cause the ball to hook (fly left) or slice (fly right), whereas if the center of gravity is situated too high or low on the club head, this will affect the ball's loft.
All skilled golfers want to own a set of clubs wherein each club has the desired feel, and further wherein all clubs have approximately the same feel so that swing technique does not need to be significantly varied from club to club. Sets of clubs which are precision-made to provide a uniform feel between clubs are highly valued and can command steep prices. However, it is difficult to obtain desirable swing weights and centers of gravity without taking special steps to modify these characteristics, particularly owing to the recent trend in golf club head manufacturing towards production of large heads made of lightweight material. When golf club manufacturers wish to modify the swing weight and center of gravity of golf clubs, this is primarily done by altering the mass distribution within the golf club head. This is generally done in two ways.
The first method of manufacturing golf club heads to meet a precisely-defined swing weight and center of gravity is to use precision casting processes to create club heads having the desired mass distribution from the moment the club head comes into existence. The club head is simply cast from molds having the desired form using highprecision casting processes. If the resulting golf club head does not meet the desired specifications, it is simply melted down and the casting process is repeated until a club head with the correct mass distribution is achieved. Because this method is quite time and labor intensive (and thus costly), it is generally used for expensive "luxury" golf clubs.
The second method is more suitable for mass production of golf club heads and is thus used in more common (and lower-priced) golf clubs. Large quantities of hollow golf club heads with weights below a desired target weight are produced by standard casting processes. Workers then sort the heads by mass into numerous classes (e.g., one class including club heads having a mass of 170-172 g, a second including club heads having a mass between 172-174 g, etc.). The workers are equipped with hand-held guns similar in structure and function to commonly-known fuel pump nozzles. Each gun's hose leads to a source of thermosetting urethane foam, and when its trigger is depressed, it ejects a charge of urethane foam having a predefined volume. Because the charges are of relatively uniform volume, they thus have relatively uniform weight, and therefore the heads within each class experience approximately the same weight increase. Descriptions of this or similar processes may be found in U.S. Pat. No. 4,438,931 to Motomiya, U.S. Pat. No. 4,523,759 to Igarashi, U.S. Pat. No. 4,553,755 to Yamada, and U.S. Pat. No. 4,803,023 to Enomoto et al. While this method is far easier and cheaper to practice than the precision-casting method discussed above, it suffers from several disadvantages.
First, because of factors such as variation in club head weight within each class, irregularity in charge volume owing to the stickiness and porosity of the foam, collection and hardening of the thermosetting material on the nozzle and corresponding constriction of the nozzle aperture, the worker's experience and accuracy in using the injection gun, and similar factors, this method results in a weight variation within each class of approximately .+-.4 grams about the desired target weight. This tolerance can cause a significant change in feel from club to club. As an example, if a set of clubs includes a #3 wood which is 4 g below target weight and a #4 wood which is 4 g above target weight, a golfer would experience a radical difference in feel when switching between the clubs.
Second, clubs in the lightest weight class (those that need the greatest weight increase) tend to get overfilled so that the hosel--the hollow stem of the golf club head--is filled with foam. Because the hosel must accommodate the golf club shaft, the hardened foam in the hosel is simply drilled out so the shaft can be inserted. These clubs then tend to be radically off-weight, and are then simply passed along to undiscerning or unsuspecting customers or are sold at a discount. On the other hand, the heads in the heaviest weight class (those that need the smallest weight increase) tend to be radically underfilled, with essentially only a small bead of foam being injected therein. Because this bead adheres to a small area in the golf club head, it tends to become unstuck after extended use and then rolls around inside of the club head. Golf clubs bearing such heads are often referred to detrimentally as "rattleheads" and are regarded as being the epitome of cheap, low-quality golf clubs.
Third, foam injection is simply not well suited for weighting of club heads wherein a very precise weight distribution is desired within the club head. In some instances, it is desirable to concentrate the weighting material at a particular area within the golf club head to achieve the desired weight distribution, e.g., to obtain the desired center of gravity. However, since foam is sticky and has fairly low density, occupying substantial volume, it is difficult for a worker to concentrate its weight at any given spot on the interior of the golf club head. Further, the guns used to inject the foam into the club heads are by no means precise, and tend to spray gobbets of foam into the interior of the golf club head at arbitrary and unpredictable locations. As a comparison, one can imagine the difficulty in trying to precisely inject a small amount of shaving cream at a particular location within a wine bottle by placing the nozzle of a shaving cream can within the bottle's mouth (and further, the bottle is opaque). While injection of foam into club heads at arbitrary locations can still provide better swing weight modification than if no weight correction is used at all, it would still be preferable to allow for the addition of weight to specific desired locations within golf club heads to allow more accurate modification of swing weight.
These difficulties have a significant economic effect on club manufacturers who are dedicated to producing quality products. When quality club sets are generated, clubs with similar swing weights and centers of gravity are matched and/or produced. Many individual clubs must be sorted through or modified before a properly matched set is generated. This takes a great deal of time, and any leftover clubs--i.e., those that cannot be properly matched into a set--are sold as individual clubs at a discount, or are donated to junior and beginning golfers. These clubs would experience increases in value of several orders of magnitude if they could simply be properly matched with other clubs.
Other weighting methods have been proposed in patents and other publications, but the inventor of the methods and apparata which this disclosure concerns has not encountered any clubs on the market which are known to be produced by these methods. Following is a description of several of these prior methods.
U.S. Pat. No. 4,502,687 to Kochevar describes inserting amounts of accurately weighed thermoplastic material within the hosel of a club head, which is then heated and positioned in a desired manner so the thermoplastic will melt and flow to the desired location within the club head. The heating and positioning may be done by use of a support fixture which incorporates a heater and which holds the club head in the desired position. The thermoplastic is then allowed to set. After thermoplastic is added, any remaining empty volume within the golf club head may be filled with foam. Preferred thermoplastics are stated to have a melting point between 230-500.degree. F.; a viscosity similar to that of honey at normal room temperature (and a time of up to 30 minutes to flow to the desired location); and a tackiness such that the thermoplastic is very sticky even when in a solid state. Other materials, e.g., metal powder, may be mixed with the thermoplastic material to obtain desired weighting properties. This method is timeconsuming, particularly owing to the lengthy heating and flowing times and the need to preweigh both the club head and a corresponding thermoplastic weighting charge, and can thus be costly. Like foam weighting, it is also difficult to have the method generate a desired weight concentration. While it sounds practicable to orient a club head so that semiliquid thermoplastic flows into a desired area, the reality is that club head interiors may have irregular surfaces, or may be contoured such that liquids will simply not flow to the desired area because of adjacent lower areas.
U.S. Pat. No. 4,824,116 to Nagamoto et al., U.S. Pat. No. 5,007,643 to Okumoto et al., and also the aforementioned U.S. Pat. No. 4,803,023 to Enomoto et al. describe the injection of foam (or foam-generating materials) interspersed with metal or glass particles. U.S. Pat. No. 5,499,891 to Nagamoto describes performing a similar method with a synthetic resin. These methods are stated to provide superior weighing ability. However, these methods suffer from the same problems experienced with use of the aforementioned foam weighting processes. Additionally, the distribution of heavier particles within foam is uncontrollable once foam enters the club head, particularly since the particles can "sink" or drift within foam prior to hardening.
There is thus a need in the art for a method and/or apparatus which will allow for precise modification of golf club swing weights without the high expense of precision casting and without the disadvantages of known foam/thermoplastic weighting processes.